I don't uderstand it. Neon never used to do it before... but anyways, heres what happend. I was using the gravel vac and I looked away for one second to make sure my dog wasn't getting into anything... I look back and poor Neon had his tail stuck in the vac. Luckily he wasn't injured, but his dorsal looks a bit funny to me... I include this because I don't know if it has anything to do with the problem.

As I added water Neon's head started twitching and shaking like crazy. I became worried, but he stopped. I noticed that every once in awhile sometimes while I watched him his head would give a little twitch.

That was yesterday. I told myself nothing had happened, then this morning he started swimming up to the surface and his head twitched, so I decided that something's wrong. But what is the problem exactly?

It sounds like he's flashing to me which is indicative of parasites on his gills. Not sure why it would start all the sudden though. I doubt it has anything to do with getting sucked into the siphon though.

I thought of that... but he isn't rubbing against anything (I haven't seen it at least) And he doesn't shake his head a lot now, it's just an occasional twitch. Do you think it's still possible that it's gill parasites? Or maybe he injured his head somehow? I don't see anything on him...

Just because a fish has parasites doesn't mean he should be rubbing against anything- flashing is just good enough, if the number of parasites is low and isn't irritating him too much. Also, fish who have parasites don't always shake their heads... it can be occasional. (And I would know, you don't know how many parasites I've had to treat )